1. Field of the Invention
The present invention relates generally to enhanced oil recovery processes, and more particularly to a huff and puff process utilizing an injected gas mixture comprising at least about 90% nitrogen by volume.
2. Description of the Prior Art
It has long been known in the oil field that in some instances the recovery of petroleum from an underground formation can be enhanced by a procedure referred to as "cyclic gas recovery" or "huff and puff".
In a cyclic gas recovery process, a chosen gas is injected into a well, allowed to soak into the formation and subsequently the gas along with the desired hydrocarbons and other fluids are produced back out of the same well into which the injection gas was injected. Thus, the name "huff and puff".
Many different gases have been utilized as the injection gas in a huff and puff process.
The general engineering theory of the performance of the huff and puff procedure, a history of its development, and a description of the various gases and gas mixtures which have been utilized is found in U.S. Pat. No. 5,725,054 to Shayegi et al. That same work is further described in paper no. SPE 36687, presented to the Society of Petroleum Engineers, Inc. in 1996, entitled "Improved Cyclic Stimulation Using Gas Mixtures", and also in the doctoral dissertation of Sara Shayegi entitled "A VALUATION OF ALTERNATIVE GASES FOR IMMISCIBLE CYCLIC INJECTION" submitted to the Louisiana State University, Department of Petroleum Engineering, in December 1997. The Shayegi references are incorporated herein by reference.
As is apparent from the summary set forth in the Shayegi references, there is a continuing search for improved injection gases to be utilized in huff and puff processes. The most commonly used gases have been steam, carbon dioxide, natural gas and exhaust gas. Previously, pure nitrogen gas has not been utilized in huff and puff procedures. The extensive literature survey conducted by Shayegi et al. as recorded in U.S. Pat. No. 5,725,054, reported at Column 3, Lines 3-4 that "no studies regarding the use of pure nitrogen for cyclic injection have been found in the literature". The laboratory tests reported by Shayegi et al. compared the use of pure carbon dioxide, pure methane and pure nitrogen, and concluded that nitrogen recovered only about one-half as much additional oil as either pure carbon dioxide or pure methane. See Shayegi et al., SPE 36687, "Improved Cyclic Stimulation Using Gas Mixtures", at Page 2.
Relatively pure nitrogen gas has been utilized in the prior art for well to well injection processes, as contrasted to huff and puff procedures. Nitrogen has been utilized in oil recovery as a dry gas or attic recovery gas in a displacement process, whereby, the nitrogen is injected into an injection well and oil is displaced to a different production well. Although there is not complete agreement by those skilled in the art as to the physical processes which are occurring in these well stimulation procedures, it is generally understood that the physical phenomena occurring during a well to well gas injection stimulation process are different from those occurring in a huff and puff process.
Additionally, the prior art has recently seen the development of improved apparatus for producing relatively pure nitrogen gas. These developments are summarized in Evison, et al. SPE 24313, entitled "New Developments in Nitrogen in the Oil Industry", 1992, Society of Petroleum Engineers, Inc. One particular new apparatus for providing purified nitrogen gas is an air separating system utilizing polymeric membranes which separate the nitrogen from the air. The description of various systems for providing purified nitrogen gases as set forth in Evison, et al. is incorporated herein by reference.
Thus, it is seen that there is a continuing need in the oil industry for further improved enhanced oil recovery processes.